841. Counting the Omer

119:12 If a person forgot to eat the afikomen and he remembered before bentching, then even if he already washed mayim acharonim or even if he already recited the zimmun, he eats the afikomen and does not need to recite HaMotzi. Even though he did not intend to eat any more, this is of no consequence because he is obligated to eat the afikomen. He does, however, have to wash his hands again, albeit without a bracha. If he only realized after bentching that he did not eat the afikomen, but it is still before reciting the bracha on the third cup, he should wash his hands, say HaMotzi, eat a k'zayis of afikomen, bentch, and then say the bracha over the third cup and drink it. If he realized his omission after saying the bracha on the third cup, he should drink the cup. If he normally bentches without a cup, he should wash his hands and eat the afikomen, then bentch without a cup. If he is normlly careful only to bentch over a cup and now cannot do so, because it would add a cup to the prescribed four, then he should not eat the afikomen and rely on the matzah that he ate earlier.

120:1 On the second night of Pesach, we start counting the Omer; this is done standing. The mitzvah is to count immediately when night begins, right after the stars come out. If necessary, one may count the entire night. We count in shul after kiddush on Shabbos and yom tov evenings in order to start the holy day earlier. (At home, one would count the Omer before kiddush because we are not permitted to eat until after we count - Mishnah Brurah 489:39.) At the conclusion of Shabbos or yom tov, we count before havdalah in order to delay the end of the holy day. When the last day of yom tov falls on a Sunday, with the result that we say both kiddush and havdalah on the same cup of wine, we count the Omer first in order to delay havdalah.